Frontends

Governing contract: C-XPILE-FRONTEND-TRAIT — Layer 3 (architectural), code lane, kind: pattern. Every frontend implements this trait; the contract pins down determinism (same input → same MetaHirModule) and the citation invariants the parsed module must carry.

A frontend reads a source file and lowers it to xpile’s canonical meta-HIR. Frontends never see other frontends; they all funnel through meta-HIR.

Status at v0.1.0

Two more frontends ship as workspace members but live in the xpile-frontend shared trait crate:

• C++ — planned
• Rust — scaffold
• Lean 4 — scaffold (sub-spec: sub/lean-bidirectional.md)

Python frontend — what’s supported

The depyler frontend is the deepest at v0.1.0. The supported subset includes:

• typed def functions
• multi-statement function bodies
• all binary operators (with Python-floor semantics for // and %)
• all unary operators
• ternary expressions
• if/elif/else chains
• function calls including self-recursion

Each emitted Rust/Ruchy/Lean function carries a // xpile-contract: C-PY-INT-ARITH citation for the arithmetic contract.

For the full list, see the CHANGELOG Python subset (live, runtime-verified) section — that’s the canonical list to avoid duplication-and-drift.

Shell frontend — what’s supported

The bashrs frontend parses a POSIX-shell subset sufficient for realistic build scripts:

• shebang normalisation
• variable assignment + expansion
• conditional file creation (mkdir -p, > file, >> file)
• pipelines
• if/elif/else
• for loops over expansions
• subprocess invocation (cmd arg1 arg2)

The supported set is locked in by the C-BASHRS-POSIX-IDEMPOTENCE contract. See the shell-roundtrip tutorial for an end-to-end example.

Calling a frontend as a library

#![allow(unused)]
fn main() {
use depyler_frontend::DepylerFrontend;
use xpile_frontend::Frontend;

let frontend = DepylerFrontend::default();
let module = frontend.parse_file("factorial.py")?;
// `module` is a `xpile_meta_hir::MetaHirModule`
}

The Frontend trait surface is intentionally minimal — see Adding a frontend for the full implementation guide.